PML, the fusion partner of RAR in PML-RARα oncoprotein which causes acute promyelocytic leukemia (APL), is involved in many growth suppressive pathways such as apoptosis, cell cycle arrest and senescence. We have demonstrated that PML is essential for the growth suppressive functions of tumour suppressors Mad and Rb. Although most of these growth suppressive functions of PML are attributed to its localization to specific nuclear domains known as PML oncogenic domains (PODs), recent findings suggest that a certain cytoplasmic pool of PML is equally involved in critical tumor suppressive pathways. We, therefore, investigated whether nuclear and cytoplasmic PML are differentially regulated by oncoprotein PML-RAR. Here we report that PML-RAR promotes selective endoplasmic reticulum associated degradation (ERAD) of cytoplasmic PML, while, at the same time, decreasing the turnover of nuclear PML protein. PML-RAR targets ubiquitinated PML protein to endoplasmic reticulum where it co-localizes with ER resident proteins ATF6, GRP78 and PDI. The PML-RAR-induced degradation of PML protein could be abrogated by proteasome inhibitor MG132, which interestingly, also promoted apoptosis of APL cells and reorganized the microspeckled like PODs to normal dot like pattern. PML-RAR expression leads to accumulation of protease resistance PML protein bands, suggesting that PML-RAR may induce mis-folding in PML protein. The PML-RAR-induced mis-folding of PML protein depends on the integrity of coiled-coil domain of PML-RAR, as a coiled-coil domain deletion mutant of PML-RAR had no effect on the conformation of PML protein. These finding identifies differential spatial regulation of PML function by PML-RAR protein.
First AACR Centennial Conference on Translational Cancer Medicine-- Nov 4-8, 2007; Singapore